Key points: How the Galactic Center was
hidden; evidence for a black hole AND for recent star formation; the circumnuclear ring

Why is it important

Conditions for star formation and evolution must be very
different in the centers of galaxies. If we lived in the center of a galaxy, there would be a million
stars between us and another star at 1 parsec distance such as alpha Centaurus. In our
actual position, alpha Centaurus is the closest star. This huge density of stars may cause
peculiar things to happen to them as they form and evolve. In addition, centers of galaxies harbor ultramassive
(up to a billion M) black holes that we
see as quasars or bright radio galaxies, exotic objects that make huge amounts of energy.
Conditions around them must be so extreme, they test our laws of physics.

In the nearest galaxies, the highest resolution pictures we
have taken show details only down to 10 to 30 light years. With perhaps 100,000,000 stars
lumped together, we can barely see even the brightest individual stars. Also, we cannot
study what happens really close to any possible black hole.

The center of the Milky Way is 100 times closer than the
next closest galactic nucleus, so we can take pictures showing details 0.03 light years in
size. This "cosmic zoom lens" makes the Galactic Center have an importance for
galaxy studies similar to the importance of the sun for stellar work.

However, extinction makes the Galactic Center inaccessible to
visible-light astronomy. It was located approximately by radio measurements, then
definitively in the starlight at 2 microns.

The central few hundred parsecs in visible light: the view is
dominated by thick obscuring clouds in the disk of the Milky Way.

Even knowing exactly where to look does not help in the visible --
there are just a few foreground stars that lie in front of the heavy clouds of obscuring
dust that hide the real center.

The region is a dramatic radio source. Here is the central roughly
hundred parsecs in the radio. The Galactic Center itself is the bright source to the lower
right. The arcs are thought to be hot gas flowing along a strong magnetic field.(From
F. Yussef-Zadeh, VLA, APOD, http://antwrp.gsfc.nasa.gov/apod/ap020521.html)

Many galaxies have supermassive black holes at their centers. Could
ours be one of those? If so, why is the black hole so dim?

Ever increasing resolution in infrared images showed the black
hole is not the energy source. The brightest source in the very high resolution
near infrared image to the right is IRS 7, a red supergiant that puts out most of its
energy in the near infrared. The other bright stars are also very young and massive. The
blue-appearing ones in the center of the image are a unique clustering of very luminous,
massive stars. Any black hole must be invisible. (image from Gemini Project). If the black hole dominated the energy of the Galactic
Center, it would be the second brightest source in the infrared image.

Here is a very deep, high resolution (1 arcsec)
X-ray image of the
Galactic Center -- the source elongated up and down just above and to the right of the
center is Sgr A*, but it doesn't stand out at all. Even in X-rays,
where we look to find stellar black holes, there is nothing to draw our attention to
a supermassive black hole here!(fromNASA/CXC/MIT/F.K.Baganoff
et al.http://chandra.harvard.edu/photo/2003/0203long/index.html)

Starting from a large scale radio image, let's zoom in on Sgr A*. We
start with a view about 2 parsecs across; if we put the sun at the center, the nearest
star would be near the edge, but in the Galactic Center the frame is filled with millions
of stars (none of which can be seen in the radio image). We zoom in by a factor of about
1,000 to see the Sgr A* radio source just barely resolved by our highest resolution radio
measurements. animation from Melia, Falcke, and Agol

Can we prove there is a black hole?????

We have measured enough velocities of stars to measure the gravitational field
accurately. Even if it doesn't make much energy, and is virtually invisible, it
turns out there is a very massive (more than 3 million sun masses) black hole right in the center.

What is happening with the interstellar gas?
What stopped the black hole from having access to lots of matter to generate
energy?

Here is a radio image of the central 5 x 5 parsecs of the Milky Way.
Sgr A* is near the center of the bright, complex source to the right. Because of its
appearance, this structure is called the "minispiral." Sgr A* is not clearly
visible on this image, which emphasizes much larger structures. (from http://www.astro.utu.fi/~cflynn/galdyn/l13.html)

Here is an image in the emission of the HCN
molecule, shown in yellow and orange. We see a ring of molecular material (diameter about
2 parsecs) surrounding the black hole.It
is shown here imaged in a radio molecular emission line, but it shows up in other ways
too. The minispiral is superimposed in blue contours. It appears to be filaments of gas
falling inward from the molecular ring.

What is the Energy Source?

The center of the molecular ring is filled with young stars.
Why did these stars form, rather than the material falling into the black hole and making
it a bright source? We do not know the answer for sure.Spectra show many of the brightest sources are
normal red supergiants. As a result, we conclude we are seeing the normal
stages of evolution of massive stars. Stars must have formed in the Galactic Center in the
last 10 million years, since that is how long it would take to evolve to the red supergiant.

So, the energy comes from young, hot stars, but
how they formed so close to the black hole is a mystery. Material that was on
its way to feed the black hole evidently became unstable and broke up just in
the nick of time to avoid falling in!